As is known in the art, in many applications it is desired to transform an output impedance from an impedance, Z0 to different impedance KZ0, where K is an integer greater than 1. One device used to perform this impedance transformation is an impedance transformer. One type of impedance transformer uses concepts described in U.S. Pat. No. 2,700,129, inventor G. GUANELLA issued Jan. 18, 1955; sometimes referred to an a Guanella transformer. Another type of impedance transformer is a Ruthroff transformer; see a paper entitled “Some Broad-Band Transformers” by C. L. RUTHROFF, Proceedings of the IRE August 1959. The basic building block of the Guandella transformer, shown in FIG. 1A, includes: a pair of transmission lines, TL1, TL2 each one of the transmission lines having a pair of electrically coupled elements, C1, C2 here shown schematically as a pair of mutual inductively coupled coils, C1, C2. A first one of the coupled elements C1 in a first one of the pair of the transmission lines TL1 has a first end E1 connected to an input terminal (IT) of the transformer and a second end E2 coupled to a first output terminal (OT1) of the transformer. A second one of the coupled elements C2 in the first one of the pair of transmission lines TL1 has a first end E1 connected to a reference terminal (RT), here system ground, and a second end E2 connected to a second end E2 of a first one of the pair of coupled elements C1 in a second one of the pair of transmission lines TL2. The first one of the pair of coupled elements C1 in the second one of the pair of transmission lines TL2 has the first end E1 connected to the input terminal (IT). The second one of the elements C2 of the second one of the pair of transmission lines TL2 has a first end E1 connected to the reference terminal RT and a second end E2 connected to a second output terminal OT2, which may be ground potential. Ideally, voltages are applied between the coupled elements C1, C2 in each transmission line TL1, TL2, and the transmission lines TL1, TL2 are interconnected together as shown to transform the input impedance Z0 at the input of the transformer to an output impedance 4Z0. It is noted that at high frequency applications, such as in microwave frequency applications, the transmission line TL1 and TL2 may be coaxial transmission lines, as shown in FIG. 1B. Here, the coupling elements C1, and C2 are the inner and outer conductors C1 and C2, respectively, of the coaxial transmission line. Here the input, or first ends E1 of the inner conductors C1 are connected to the input terminal IT and the input, or first, ends E1 of the outer conductors C2 are connected to the reference terminal RT. The output, or second end E2 of the inner conductor C1 of transmission line TL1 is connected to the first output terminal OT1 and the second end E2 of the outer conductor C2 of the second transmission line TL2 is connected to the second output terminal OT2. The second end E2 of the outer conductor C2 of the first transmission line TL1 is connected to the inner conductor C1 of the second transmission line TL2, as shown. It is also noted that the basic building block can be used to form other networks such as baluns.
This basic building block can be multiplied and arranged to provide higher impedance transformers. For example, a 5:1 Guanella impedance transformer providing a 25Z0 impedance transformation (where Z0 is the input impedance of the transformer) is shown in FIG. 1C. Here, the Guanella impedance transformer is fed by an amplifier having an output impedance Z0. The output of the amplifier is fed to a 5:1 power divider or splitter having here, in this example, five output coupled to the inputs of five transmission lines, TL1-TL5, respectively, here represented as a pair of mutually inductively coupled coils, C1, C2. More particularly, the upper one of coils, C1, in each one of the transmission lines TL1-TL5 has an input, or first, end E1 connected to the output of the amplifier and the lower one of the coils, C2, in each one of the transmission lines TL1-TL5 has an input, or first, end E1 connected to system ground. The output, or second, end, E2, of the lower coil, C2 in transmission line TL1 is connected to an output end E2 of the upper coil C1 in the next one of the transmission lines, here transmission line TL2; the second end, E2, of the lower coil, C2 in transmission line TL2 is connected to an output end E2 of the upper coil C1 in the next one of transmission line, here transmission line TL3; the second end, E2, of the lower coil, C2 in transmission line TL3 is connected to an output end E2 of the upper coil C1 in the next one of the transmission line, here transmission line TL4; the second end, E2, of the lower coil, C2 in transmission line TL4 is connected to an output end E2 of the upper coil C1 in the next one of the transmission lines, here transmission line TL5. The second output end E2 of the upper coil C1 of transmission line TL1 provides an output terminal of the Guanella impedance transformer and the second output end E2 of the lower coil C2 of transmission line TL5 is connected to system ground, as shown. The five outputs of the power divider are in-phase with each other; that is, they have the same electrical length or time delay from the output of the amplifier to the first end E1 of each the upper one of coils, C1, in each one of the transmission lines TL1-TL5. With such an arrangement, the voltage produced across the outputs ends E2 of coils C1, C2 of each of the transmission lines, TL1-TL5 will be V1-V5, respectively, as indicated. See also, for example: U.S. Pat. No. 7,495,525, issued Feb. 24, 2002, Ilkov et al.; U.S. Pat. No. 6,756,874, Buckles et al., issued Jun. 29, 2004 and, Power Combiners, Impedance Transformers and Directional Couplers by Andrei Grbeanikov, December 2007 High Frequency Electronics Copyright © 2007 Summit Technical Media, LLC.
Further, by properly adjusting, or minimizing, the time delays (TD1-TD5) of the connections (in effect the length of the connectors) between the second ends E2 of one of the transmission lines to second end E2 of the next one of the transmission lines, VIN=V1=V2=V3=V4=V5 and thus, the output voltage of the transformer is 5*VIN. The current IIN is split equally among the ends E1 of coils C1 of TL1 thru TL5 such that each current I1=I2=I3=I4=I5=IOUT=IIN/5. As a result, the output impedance of the amplifier, Z0,IN=VIN/IIN, will be transformed by the Guanella impedance transformer to output impedance Z0,OUT=VOUT/IOUT=(5*VIN)/(IIN/5)=25Z0,IN. In many high frequency applications, such as in microwave applications, the transmission lines are coaxial transmission lines, as shown in FIG. 1D. FIG. 1D shows a coaxial transmission line implantation of the 5:1 Guanella impedance transformer shown in FIG. 1C. In order to improve performance, primarily bandwidth, the coaxial transmission lines are enclosed in a ferrite core, not shown. The real world implementation of the transformer is impaired by certain real-world features in its layout; namely the above-mentioned time delays developed in the lines connecting the transmission line sections to one another.